1. Field of the Invention
This invention relates to battery powered computer systems and, more particularly, to methods and apparatus for reducing the power requirements of computer subsystems associated with a battery powered host computer.
2. History of the Prior Art
It is very difficult to provided sufficient energy to operate a battery powered electronic device for any length of time. Most people are familiar with the constant need to replace the batteries used by radios and tape recorders and to recharge the batteries of those electronic devices which are rechargeable. Capable methods of providing power to operate such devices for extended periods have not yet been devised. Batteries having a useful lifespan are simply to heavy for many portable devices.
This is especially true of portable computers in which a number of components provide a constant drain on the batteries during operation. For example, many portable computers utilize large electro-mechanical hard disk drives in order to store the large programs which have become prevalent and large displays in order to display those programs in operation. The operation of these subsystems requires substantial amounts of energy. Certain portable computers now utilize color displays which require even larger amounts of energy than do the displays of typical portable computers. Because of these energy-consuming components, it is unusual to find a portable computer which is able to operate for longer than two hours on batteries. This is much less time than most longer air flights, an application in which such computers see a great deal of battery use.
For this reason, designers have been providing methods and apparatus for conserving the energy utilized by portable computers. In general, these have encompassed arrangements for turning off the display and the electro-mechanical hard disk drive after some period of time. These solutions have provided substantial savings in energy and have helped portable computers reach the present state in which over two hours of operating time is realized.
One problem with these power conservation methods is that the amount of energy which can be saved with such components is limited. Moreover, the particular methods must be carefully designed or they are likely to utilize more energy than they save. For example, an electro-mechanical hard disk drive requires a substantial amount of energy and time to spin up to the active condition from an off condition. If such a disk drive is constantly turned off after each use, then the energy consumed in bringing it back into operation may be greater than the energy saved by turning it off. It is for this reason that some interval is usually interposed before a disk drive is powered down from its last use. It is also for this reason that the amount of energy which can be saved with such components is limited.
To solve the problem of energy use and for other reasons, forms of long term storage other than electro-mechanical hard disk drives have recently been used in portable computers. One of these forms of long term storage is a flash EEPROM memory array. A flash EEPROM memory array includes a large plurality of floating-gate field effect transistors arranged as memory cells in typical row and column fashion with circuitry for accessing the individual cells and placing the memory transistors of those cells in one of two memory conditions. A flash EEPROM memory cell, like a typical EPROM cell retains information when power is removed but does not require removal from the system to be erased. Flash EEPROM memory has a number of features which adapt it to use as long term memory. It is light in weight and occupies very little space. It is especially rugged. It will withstand without adverse effects repeated drops each of which would destroy a typical electro-mechanical hard disk drive. Because it is electronic rather than electro-mechanical, a flash EEPROM memory consumes less energy than electro-mechanical disk drives.
Although flash EEPROM memory arrays use less energy than the typical electro-mechanical hard disk drive in active operation, it would be very useful were it possible to further reduce the energy required for the operation of these subsystems and other subsystems which utilize internal microprocessors, logic circuitry, and random access memory.